Gravity meter



Nov. ll, 1941. n. H. cLEwELI.

GRAVITY METER Filed July 3, 1940 4 Sheets-Sheet l 3mm v@J1/Zaini, (few/lSheets-Sheet 2 Uhm,

D. H. CLEWELL GRAVITY METER Filed July 3, 1940 l l, m

` Nov. 11, 1 941.

Nov, El, w43. u. H. CLEWELL GRAVITY METER Filed July 3, 1940 4Shsees-SheeI 3 /QMMS W Nov. El, 1941.

GRAVITY METER Filed July 3, 1940 4 Sheets-Sheet 4 D. H. CLI-:WELL2,262,565

Patented Nov. 11, 1941 GRAVITY METER Dayton H. Clewell, Dallas, Tex.,assignor, by

mesne assignments, to

Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a

corporation of New York Application July 3, 1940, serial n3431133 (ci.ass- 1.4)

Claims. I'his invention relates generally to geophysical instruments ofthe type used in locating abnormalities such as discontinuities that areocca-AV sioned by faults, anticlines'and structures such as salt domes.More particularly, this invention relates to a method and apparatus formeasuring directly variations in gravitational force from point to pointover the earths surface.

It has long been known to geologists and others skilled in the art ofgeophysical prospecting that variations in gravitational force frompoint to point over the earths surface are directly related to thedisposition of the substrata with respect to the earths surface;therefore by measuring the variations in gravitational force from pointto point over an area of the earths surface and plotting these data onmaps in the form of contours, a map is formed which simulates thecontours of the subsurface strata. Such instruments have been found tobe quite reliable and they are being used in the industry successfullyin conducting the reconnaissance geophysical surveys.

Due to the fact that fractional variations in gravitational force beingmeasured are sometimes no more than 'I a very delicate instrument ofextreme accuracy is required. These instruments must be accurate withinone part in ten million. .i The instant invention is directed to such aninstrument having extreme flexibility in that means are provided fornulling the instrument by changing the lever arm through which apretensioned main spring acts upon the pivoted mass without varying thesensitivity of the instrument itself. Additional features included inthe design of this instrument reside in the provision of novel means forcritically damping the mass; means for rigidly clamping the mass whilethe instrument is being moved from one location to another; means forvarying the position of the anchored end of the pretensioned main springto effect either a change in the sensitivity of the instrument or toeffect a change in the'lever arm through which the main spring acts uponthe mass; and an optical system by means of which displacements of themass can be directly observed.

Therefore, the primary object of this invention resides in the provisionof a novel method and apparatus whereby the variations in gravitationalforce from point to point throughout an area on the earths surface canbe measured by a null system.

An additional object of this invention resides in the provision of meanswhereby the sensitivity of the instrument aswell as the effective leverarm through which the main pretensioned spring.

acts can be varied by movement of the anchored end'of the pretensioned'main spring.

Still another object of this invention resides Y in the provision ofnovel clamping means whereby the mass is normally rigidly clamped.

This invention further contemplates means for critically damping themass to cause it to come to rest at once so that as short an interval oftime as possible will be consumed in making a reading at a location.

Still another object of this invention resides in the provision of novelmeans whereby the displacement of the mass can be observed.

Another object of this invention resides in the provision of means.whereby the force required to change the lever arm through which themain pretensioned spring is acting to eiect nullingof the instrument canbe measured directly.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when considered withthe drawings in which:

Figure 1 is a side elevation of the instrument shown partly incross-section disclosing the various elements in operative relationship;

Figure 2 is a plan view of the apparatus taken along the line 2--2 ofFigure 1, showing some of the elements in section;

Figure 3 is an end elevation of the device taken along the line 3--3 ofFigure 1, showing the apparatus within the case;

Figure 4 is a perspective view of the novel pivot means for the mass;

Figure 5 is a detailed sectional View of the mass clamp actuating means;

' Figure 6 is a side elevation of a gravity meter similar to that shownin Figure 1, showing a modified form of null system;

Figure 7 is a side elevation of asimilar type of gravity meter differingfrom Figures 1 and 6 in that a system of levers are used to displace themain pretensioned spring to eiect nulling of the system; and

Figure 8 is afragmentary sectional view taken along the line 8-8 ofFigure '7, showing the sysf tem of levers by means of which the body ofthe be a at leaf spring (Fig. 4) that is secured to the support I2 bymeans of suitable screws or bolts I6. The ends of the bar I5 carryclamps to receive ends of delicate leaf springs I1 and I8. Springs I1and I8 are disposed parallel to each other andhave their lower endssecured by means of clutches I9 and 20 respectively, to a rod 2l that isrigidly secured to the mass I4. Rod 2| is held in xed relationship withmass I4 by means of the clutch 22 and its accompanying screws or bolts23. Springs I1 and I8 are :made of very thin resilient material andserveonly as means through which the mass I4 can pivot. The mass` I4 issubstantially L-shaped and extends from the enlarged clutch portion 22to an enlarged portion 24 which formsits angle. The enlarged portion 24is provided with surfaces 25 and 26 that are adapted to act as bearingsurfaces for clamping means. An upright portion 21 of reduced crosssection extends from the enlarged portion 24 of the mass to a point nearthe top 28 of the casing I0. Anchoring means 29 are secured to and areadapted to be carried by this upperend of'the mass by means of which apretensioned'horizontally disposed coil spring 3D can be secured at oneend thereto. The other end of 'the coil spring 30 is secured to ananchor 3| which-is provided with novel adjusting means to be describedin detail later in the specification. Y I'he mass I4 on each of its arms21 and 32 is provided with minor masses 33 and 34 each of which isadapted to be adjusted along its respec- `tive arm to change the centerAof gravity of the mass. These minor masses are held in adjusted positionon arms.,21 and 32 by means of set screws 35 and 36 respectively. Thesemasses, however, when properlyadjusted in the laboratory, seldomrequire' further adjustment.

The'flarms 21 and 32 of the mass I4 and the pretensioned coil spring 30lie in the same vertical plane and the mass is adapted to move only inthe vertical plane about an eiective axis which would lbe a line passingtransversely through the centers of springs I1 and I8. The upper end ofthe arm 21 of the mass has secured thereto a T-shaped member 31. Securedto the oppositely disposed arms ofA the T-shaped member are cups 38 eachof which is adapted to rest within a second pair of cups 39 to produce adash pot. These cups are xed to a support 46 carried by the side II ofthe casing IIJ`. The cups 39 are provided with inner sealed drum shapemembers 4I that are concentrically disposed in spaced relationship withthe cups 38. The drums 4I, the cups 38 and the cups 39 are so spacedconcentrically that very small annular air passageways are formed. Theelements of this novel damping means are so arranged that under normaloperation of the instrument, the cups 38 and 39 will never contact eachother, but will develop on oscillation of the mass, movement of airthrough the annular. passageways that will immediately bring the mass torestbecause of viscous resistance of the air to flow through saidpassageways. i

The tension on the main pretensioned spring can `be adjusted by thenovel adjusting means provided at the end of the spring opposite themass.i 'I'here is provided at this end of the spring dual-adjustingmeans that are adapted to move the anchor 3|' for the spring30 in any desired direction. The anchor 3I'being secured to and adaptedto move with element 42 will on movement of the screw 43 eifectadisplacement of the anchor in a plane at substantially a 45 angle to theaxis of the spring. Movement of screw 43 is eiected from a point outsideof the casing I0fby means of the manipulating knob 44, rod 45, worm 46and Worm gear 41. Movement of the anchor along a line that is at rightangles to that movement effected by screw 43 can be effected by thescrew 48 which engages the sliding member 49. The sliding member 49 isslidably retained within a collar 50 which is secured to or madeintegral with a support 5I. Support 5I is common for both of theadjusting means for the anchor 3|. The screw 48 is manipulated to effecta movement of the anchor 3I through means of the worm gear 52, the worm53, the rod 54 and the knob 55 from a point without the casing I0. Sincethe sliding member 50 is adapted to move in a line at right angles tothe direction of movement of element 42 it is obvious that the anchor 3|for the pretensioned spring 30 can be moved in any desired direction toeffect a change in the sensitivity of the instrument or to' calibratethe instrument. In some cases, if desired, these two adjusting means canbe operated together to ef- I fect nulling of the instrument. It canreadily be seen that by changing the position of the anchor 3ll for thepretensioned spring 30 and the position of the small masses 33 and 34 onthe arms 21 and 32 respectively of the mass, a wide range of adjustmentis afforded this instrument. For a vparticular area that it is desiredto survey by means of the gravitational method adjustments of the deviceto the desired range are normally made by means of the knob 45. Theseadjustments when made, move the anchor of the spring 30 in such a mannerthat the mass is `brought within the operating range. In order toascertain when the mass is within the operating' range of the device,that is, the range throughout which y the instrument can be nulled, anarm 56, that is secured at one of its ends to the arm 21 of the mass, isprovided at its outer end with a prism 51 having lines representing ascale 58 etched thereon. 'I'his prism and the arm 56 is adapted to movewith the mass when displaced.

Therefore, if a source of light from a lamp 59 is projected through alens 60, then through a second lens 6I onto the face 62 of the prism 51,this light will be reflected at right angles by the prism through atelescope 63 onto a second prism 64. This light striking a second prism64 will be reected back through a transparent scale 65 to the telescope66. By means of the telescope 66, the scale 65 and the scale 58 carriedby the prism 51, it is possible for the observer to tell when the massis at its zero position. The zero position is the adjusted position forthe instrument at its base station. In conducting gravitational surveysthe mass through means of the indicator or pointer comprising the arm56, the prism 51 and its scale 58, is brought to approximately zeroposition and then further adjustment is made to 4 the position of themass by Vernier means to bring the scale reading to exactly zero. ThisVernier adjustment is made by changing the effective lever arm throughwhich the pretensioned spring 36 acts upon the mass. To accomplish thisa light spring 61 similar to that used in conjunction with the balancewheel of a Watch is secured at its outer end 68 tothe spring 30 at avpoint intermediate the ends thereof. The inner end of the light spiralspring'61 is secured to an axle formed by a rod 69 in such a manner thatrotation of the rod will apply or remove tension from this spring.Variation of the tension in the spring 61 will effect a displacement ofthe body of the spring 30 to eifect a change in the veffective lever armthrough which spring 30 acts upon the mass. The tension in spring 61 isvaried by rotating the rod 69 by means of knob 10. The amount of`rotation imparted to the rod 69 to effect a change of tension in thespring 61 is indicated by means of the pointer 1| carried by the innerend of rod 69, and the scale 12 over which the pointer 1| rides.

Due to the delicacy of instruments of this kind it is necessary thatmeans be provided whereby the mass can be rigidly clamped while theinstrument is being subjected to jars or movements of any characterwhich would change the physical characteristics of the sensitiveelements. To this end there is provided a support 13 which is secured tothe side of the casing I0. Support 13 is so formed that it presents aface 14 to the mass that is parallel to the face 25 of the enlargedportion of the mass. Extending substantially horizontally from the face14 of the support, are three'iingers 15 which are adapted to` make pointcontact with the face 25 of the enlarged portion of the mass when themass is clamped 'in inoperative position. Faces 25 and 26 of theenlarged portion of the mass are made parallel. Face 26 is adapted to becontacted by a point 16 carried by a plunger 11 to clamp the massagainst the fingers 15. Plunger 11 is slidably held within a cylindricalcasing 18. Casing 18 is made integral with or secured to an arm 19 thatextends to side of the casing where it is fastened by suitable screws orbolts. Plunger 11 is provided with a stem 80 that extends upwardlythrough the cylinder 18 to a point outside the cylinder. A guide plug 8|threadedly engages the inside and forms a closure for the upper end ofcylinder 18. This plug 8| is centrally bored to provide a passageway forthe stem 80 of the plug 11. Disposed within the cylinder, concentricallywith the stem 80 between the plug 8| and the plunger 11 by bearingagainst their inner faces, is acoil spring 82 which normally tends toforce the plunger'11 to its outermost position. Movement of the plunger11, however, is limited and controlled by a yoke 83 that is pivotallysecured to a projecting shoulder on the under surface of the cylinder 18near its lower end by means of a pin 84. The two arms 85 and 86 of yoke83 extend to diametrically opposite sides of the lower end of cylinder18. The ends of the arms of the yoke are provided with openings adaptedto receive pin 81 carried by the plunger .11. The pin 81 extends fromthe plunger through elongated openings in the cylinder 18 to pointsoutside of the cylinder and extend through the openings in the arms ofthe yoke. ment of the end 90 of the yoke will effect a pivoting of theyoke about the pin 84 as an axis to cause a reciprocation of the plunger11 within the cylinder 18. Movement of the end 90 of the yoke iseffected by means of a rod 9| a'nd the adjusting screw 92. Rod 9| isjournaled in. a

l bearing 93 in such a manner that vertical move- To clamp the mass ininoperative position,v

screw 92 is backed up to remove pressure from the end 90 of the yoke83.k The spring 82 causes the plunger 11 to move downwardly and out ofthe cylindrical housing 18 until the point 16 contacts the face 26 ofthe mass and forces the face 25 of the mass firmly against the ngers 15.'I'he mass is held in this clamped position when there is likelihoodthat the instrument as a whole will be moved or disturbed in such amanner that there is a likelihood of changing the physical constants ofthe delicate elements comprising the working parts of the instrument.When the instrument has been properly set up and leveled at a stationwhere it is desired to measure the force of gravity or any variation inthe force of gravity at that point, as compared to another point, thescrew 92 is screwed down to appllyil pressure through the rod 9| on theend 90 of the yoke 83 to cause the plunger 11 to be moved back from themass into the cylindrical case 18, a distance sufciently far that themass Will swing free of the point 16 and the fingers 15.

In Figure 6 .there is shown a modiiication of the null system disclosedin Figure 1, differing therefrom in that a light coil spring 91 is usedto displace the main pretensioned spring 30 instead of the spiral typeof spring 61 shown in Figure 1. The lower end of the light coil spring91 is secured to the body portion of the main pretensioned springintermediate the ends thereof, while the upper end of spring 91 isaxially secured to an adjusting screw 98, which on rotation will ncreaseor decrease the tension in spring 91 to displace the body of the mainpretensioned spring 30. This effects a change in the effective lever armthrough which the main spring acts upon the mass |4. Adjusting screw 98carries iixed thereto an indicator 99 that is adapted to cooperate witha scale |00 to give a reading of the amount of effort exerted indisplacing the body of the main pretensioned spring 30. The scale |00can be calibrated in terms of gravitational tov Therefore, moveforce andreadings made of the variations of gravitational force from point topoint by noting the amount of effort exerted on spring 91 or themovement of the screw 98 necessary to displace the body portion of themain spring 30, an amount sufficient to null the system or return themass to its zero or base station reading.

I'his null system like that discussed in connection with Figure 1 has aninherent advantage over other null systems in that nulling isaccomplished Without appreciably changing the sensitivity of theinstrument.

which is adapted to cooperate with a scale |02 `to indicate thedisposition of the mass, this is merely for the purpose of illustrationand it is obvious to those skilled in the. art that an optical systemsimilar to that shown in Figure 1 could be used without departing fromthe spirit of the invention.

In Figure 7 there is shown still another modication of the null system.In this form of the invention, as well as in the other two describedabove, nulling is accomplishedby displacing the body portion of thepretensioned spring 30 to change the effective lever arm through whichit acts upon the mass. In the system disclosed in Figures 7 and 8, aplurality of leverswith connecting linkage are used.

Referring `to Figure 8, a support |03, a fragment of which is shown, isappropriately secured to an inside wall of the casing l0. Lever |04by'means of a light leaf spring |05 is pivotally secured at one end tothe support |03 by means of a screw |06. Lever |04 has its end |01 whichis adjacent the pivot, bent to form an arcuate Although there is shownan indicator |0| carried by the arm 21 of the mass .of the casing |0.

The arcuate seat |08 is formed in 1everl04 very near its pivot point sothat a relatively large movement of end "|09 of lever |04 will eiectonly a very small movement of the body of spring 30. Due to the factthat it is only necessary to move spring 30 a very small fraction of aninch to effect a nulling of the instrument, the magnication of thismovement must be very great. In order to further magnify the movement ofspring 30, end |09 of lever |04 is connected by means of a link l to asecond lever Ill. III is provided with a pivot ||2 very near the pointwhere it is secured to link ||0, thereby forming a very short lever arm||3 on the link side of the pivot. The distance from the pivot to lend||4 of the lever is made as great as possible in order to obtain thenecessary magnication of the displacement of the mainspring 30. End IIIof lever is biased upwardly into contact with an adjusting screw I I5 bymeans of a light spring H6. The lower end of spring ||6 is secureddirectly to lever and the upper end of the spring is anchored to theinner surface Adjusting screw H5 is provided with an indicator orpointer that is adapted to cooperate with a scale ||8 to indicate i theamount of rotation of the screw necessary y, an area by measuring thevariations in gravitational force from point to point over the area, thereadings made directly from scale H8 can be plotted to give a contourmap which would simulate the elevation or existent conditions of thesubstrata.

Many other detailed features that may be added asl renement to thegravity meter illustrated' will immediately become apparent to thoseskilled in the art, and it is to be understood that the scope of thisinvention includes the principles of this invention regardless ofwhether or not 'these numerous additional features are incorporated.

I claim: L

1. In a gravity meter having a mass pivoted for rotation in a verticalplane, a pretensioned spring for elastically balancing the weight of themass, an indicator carried by the mass, xed means adapted to cooperatewith the indicator for measuring the displacement of the mass, a helicalspring secured to the pretensioned spring at a point intermediate theends thereof, a support, and means for adjustably securing the free endof the helical spring to the support whereby achange of tension in thehelical spring will laterally displacel the body of -the pretensionedspring to effect a change in the lever arm' through which thepretensioned spring acts upon the mass to displace the mass an amount-suicient to null the instrument.

2. In a gravity meter having a mass pivoted for rotation in a verticalplane,` a pretensioned spring for elastically balancing the weight ofthe mass, an indicator carried by the mass, fixed means adapted tocooperate with the indicator for measuring the displacement of the mass,a

coil spring secured to the pretensioned spring at a point intermediatethe ends thereof, a support, and means for adjustably securing the freeend of the coil spring to the support whereby a change of tension in thecoil spring will laterally displace the body of the pretensioned springto effect a change in the lever arm through which the pretensionedspring acts upon the mass to displace the mass-an amount sufficient tonull the instrument.

3. In a gravity meter having a mass pivoted for rotation in a verticalplane, Va pretensioned spring for elastically balancing the weight ofthe mass, an indicator carried by the mass, xed means adapted tocooperate with the indicator for measuring the dislpacement of the mass,elastic means secured to the pretensioned spring at a point intermediatethe ends thereof and extending at substantially right angles to thepretensioned spring, a support, and means vfor adjustably securing thefree end of the elastic means to the support whereby a change of ten'-sion in the elastic means will laterally displace the body of thepretensioned spring to effect a change in the lever arm through whichthe pretensioned spring acts upon the mass to displace the mass anamount sulicient to null the, instrument.

4. In a gravity meter of the type used in conducting geophysical surveyshaving a support, a mass, means for pivotally mounting the mass on thesupport, elastic means for supporting the mass against they action ofgravitational forces, means for indicating Ithe disposition of the'massrelative to a part of the support and means for nulling the system tomeasure `the variation in gravitational vforce from point to point overthe earths surface, said nulling means comprising elastic means forexerting a minor effort in a direction transversely of the axis of thefirst elastic means to transversely displace the rst elastic means toeffect a variation in the lever arm through which the rst elastic meansacts upon/'the mass, whereby the mass is caused to move in a verticalplane about its pivot to a predetermined'position, and means forindicating the amount of minor effort lexerted on the rst elastic meansto null Vthe system.

5. In a gravity meter of the type used in ccnducting geophysical surveyshaving. a support, a mass, means for pivotally mounting the mass on thesupport,v elastic means for supporting the massA against the action ofgravitational forces, means for indicating the disposition of the massrelative to a part of the support and means for nulling the system tomeasure the variation in gravitational force from point to point overthe earths surface, said nulling means comprising means for exerting aminor effort in a direction transversely fof the axis of theelasticmeans 'to transversely displace the elastic position, and means forindicating the amount" of minor eiort exerted on the elastic means tonull the system. Y DAYTON H. CLEWELL.

